1. Field of Invention
The present invention relates to an apparatus and method for the measurement of inductance. More precisely the present invention relates to an apparatus and method for the measurement of inductance of vehicles moving in a traffic lane using permeability-modulated carrier referencing.
2. Description of the Related Art
It is well known in the prior art to measure the inductance of a wire-loop, which is part of the frequency determining circuit of an inductance-capacitance-resistance (“LCR”) oscillator, using frequency-counting techniques. Typically, the number of zero-crossings per time increment of the voltage across the terminals of the LCR capacitor is counted. Because the frequency of the LCR oscillator is inversely proportional to the square root of the inductance of the LCR circuit, changes in the inductance of the wire-loop are reflected in changes of the number of zero-crossings counted per time increment.
The Class-C wire-loop oscillator described in U.S. Pat. No. 3,873,964 issued to Thomas R. Potter on Mar. 25, 1975 is typical of LCR oscillators used in the prior-art. When a vehicle passes over a wire-loop connected to a running LCR oscillator, the metal of the vehicle changes the permeability of some of the space surrounding the wire-loop causing modulation of the carrier wave generated by the LCR oscillator. Changes in the inductance of the wire-loop caused by the vehicle are thus superimposed onto the LCR oscillator's carrier wave, yielding a permeability-modulated carrier. Next, the inductive signature is retrieved from the permeability-modulated carrier. One method of demodulating the carrier is the use of frequency counting techniques, such as with “signature cards” which are commercially available from 3M Corporation and Peek Traffic. The signature cards offer approximately a 100 Hz-sample rate, which is not fully adequate for demodulating the inductive signatures of vehicles moving at highway speeds.
Another problem associated with the measurement of inductance in a wire-loop is crosstalk. Crosstalk between two or more wire-loops is a result of inductive coupling between the wire-loops, which results in energy transfer between the wire-loops when a changing current is flowing through them. If two wire-loops are operating at nearly the same frequency, then the energy transfer can result in an exaggerated buildup, or stagnation, of transferred energy in one LCR circuit, and a corresponding exaggerated energy depression in the other. This can cause the carrier waves of the two circuits to become entrained with each other in a more-or-less fixed phase differential and effectively eliminates the ability of the wire-loops to detect vehicles independently of each other. Typically, an inductive coupling coefficient of only a few percent is sufficient to cause complete entrainment. In prior-art vehicle detectors, carrier wave entrainment due to crosstalk is partially avoided by operating the oscillator circuits associated with the wire-loops at different frequencies, typically by varying the value of the capacitance, C, of the LCR circuit. This can prevent stagnation and entrainment, but does not address the underlying errors induced into each detector by the energy transfer due to mutual inductive coupling.
Accordingly, there is a need for an apparatus and method for measuring the changes in the inductance of a wire-loop caused by a vehicle traveling along a monitored roadway. The apparatus and method need be capable of measuring changes in the inductance of a wire-loop caused by a vehicle traveling at highway speeds. Further, the apparatus and method should be capable of measuring inductance without attempting to identify frequency changes. Finally, there is a need for an apparatus and method capable of measuring inductance using multiple inductive sensors without significant errors resulting from crosstalk.
Therefore, it is an object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle traveling along a monitored roadway.
It is another object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle traveling at highway speeds.
It is a further object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle and producing an inductive signature for that vehicle.
It is yet another object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle and producing an inductive signature of that vehicle by referencing a measured voltage to a permeability-modulated current carrier wave.
A still further object of the present invention is to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle using multiple inductive sensors without significant errors resulting from crosstalk.
Another object is to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle which does not need to be installed in the driving surface of a roadway.